1. Field of the Invention
The present invention relates to ferromagnetic thin films having perpendicular anisotropy. More particularly, it relates to ferromagnetic thin films utilizing as the ferromagnetic material a Manganese-Gallium compound having the formula .delta.-Mn.sub.1-x Ga.sub.x, where X=0.4.+-.0.05. Such thin films are useful in vertical magnetic/magneto-optic recording.
For a ferromagnetic thin film the anisotropy energy includes contributions from surface or interface anisotropy, volume magnetocrystalline anisotropy and the demagnetizing field or shape anisotropy. In general for thick films (.about.few nm for transition metals) , the demagnetizing field is predominant and the magnetization lies in the plane of the film. For ultrathin films and multilayers (t.sub.m &lt;1 nm) the surface/interface contribution, proportional to t.sub.m.sup.-1, can overcome the shape anisotropy and result in a spontaneous magnetization perpendicular to the film. Alternatively, the magnetocrystalline anisotropy can dominate provided the material exhibits strong uniaxial anisotropy and is grown such that the easy axis is oriented along the film normal. The perpendicular anisotropy lends itself to vertical magnetic/magneto-optic recording and epitaxial growth on semiconductor substrates could lead to novel devices.
From a technological point of view, the development of a suitable material for magneto-optic recording imposes a set of stringent requirements that includes perpendicular anisotropy to achieve high bit densities (with vertically magnetized domains), a high coercivity to support micron or sub-micron sized domains, a square hysteresis loop for low writing noise, maximum Kerr rotation angle and optical reflectivity for best magneto-optic signal and a Curie point in the range 180.degree.-300.degree. C. for stable domains with good writing sensitivity. (Materials for magneto-optic data storage, C. J. Robinson, T. Suzuki and C. M. Falco editors, MRS Symposium Proceedings, Vol. 150 (1989).) In addition, it is helpful if the material is corrosion and oxidation resistant, thermally stable and easy to prepare. Finally, epitaxial growth on an appropriate semiconductor substrate with minimal inter-diffusion and interfacial reactions is desired. This would made the exciting possibility of the integration of magneto-optic and semiconducting devices a reality.
2. The Prior Art
Even though a number of thin film materials are being considered and developed none of these materials satisfy all these requirements and each one of the potential candidates has some major deficiency. For example, MnBi exists in two crystallographic phases and the more desirable phase is unstable; amorphous rare earth-transition metal alloys, though most promising, have intrinsically weak magneto-optic activity at wavelengths of interest and the alternative polycrystalline Bi/Ga doped RE-iron garnets have significant grain noise and irregular wall strength.
Ultrathin multilayers (Co/Pt etc.) are currently being developed to overcome some of these limitations by exploiting their surface and interface anisotropy properties. (W. B. Zeper, F. J. A. M. Greidanus, P. F. Carcia and C. R. Fincher, J. Appl. Phys., 65, 4971 (1989).) Being nanostructured materials, they involve complicated processing. In addition, the origin of perpendicular anisotropy in these films is poorly understood and has been variously attributed to interface anisotropy, compound formation at the interface, strain, etc., and this limits further development of this material. (P. Bruno and J.-P. Renard, Appl. Phys., A49, 499 (1989).)
A viable alternative to achieving these goals is to carefully select a ferromagnetic material that in the bulk exhibits uniaxial anisotropy (in addition to the right combination of T.sub.c, H.sub.c, and saturation magnetization), and grow it epitaxially (good lattice matching is important) on appropriate substrates (preferably on semiconductors for device integration) such that the magnetization is along the film normal. Using this approach, films of .tau.-MnAl have been grown on GaAs. (T. Sands, J. P. Harbison, M. L. Leadbeater, S. J. Allen, G. W. Hull, R. Ramesh and V. G. Keramidas, Appl. Phys. Lett., 57, 2609 (1090).) Even though they have reported comparable properties there are three major difficulties: (i) the .tau.-MnAl phase is metastable, (ii) the growth requires deposition of a complex AlAs inter-diffusion barrier and (iii) the properties are optimal for a film thickness of 10nm and deteriorate rapidly above 20nm. (T. L. Cheeks, M. J. S. P. Brasil, T. Sands, J. P. Harbison, D. E. Aspnes and V. G. Keramidas, Appl. Phys. Lett., 60, 1393 (1992).)
I have now discovered that Mn.sub.1-x Ga.sub.x X=0.4.+-.0.05 is the most promising candidate to achieve these stringent mix of properties. In the bulk it is tetragonal (a=2.75 .ANG., c=3.542 .ANG.) and ferromagnetic (H.sub.c (77K)=1.15-4.9 kOe, T.sub.c =658-748 K, .sigma..sub.s (77K, 10 kOe)=48-31 G cm.sup.3 /g). (T. A. Bither and W. H. Cloud, J. Appl. Phys., 36, 1501 (1965).) In addition, growth of the c-axis oriented film on a GaAs (100) substrate for device integration would be both chemically and crystallographically favorable.